Most IC devices generally include numerous transistors such as MOSFET. They are required to have limited power consumption. This is true particularly when they are used in portable electronic devices.
FIG. 1 shows schematically a structure of a typical IC device configured to operate at a reduced power. Block 10 represents a transistor circuit block involving many transistors like CMOSFETs. The IC device as shown in FIG. 1 further includes a power switch consisting of a P-type MOSFET 11 and an N-type MOSFET 12 for turning on and off power to the circuit block 10. They are connected between a power supply having a potential E1 and the ground of potential Egnd. The supply voltage Vcc amounts to the potential difference between the two potentials.
In operation, transistor circuits inside the block 10 are switched at high frequency. In order to reduce power consumption by the circuit block 10, the supply voltage Vcc is set as low as 2 V say. In a transistor circuit, reduction of the supply voltage Vcc is effective in reducing its power consumption, since power consumption is proportional to the square of the supply voltage. However, when the supply voltage is reduced, operational speed of the transistor circuits in the block is also reduced, making an appreciable delay in the device.
One way to circumvent such delay is to reduce the threshold voltage of the transistors (e.g. CMOSFETs) of the circuit block 10 in correspondence with the reduction of the supply voltage Vcc. This reduces only the power consumption without reducing the operational speed of the block 10.
However, with a reduced threshold voltage as mentioned above, leak current through the MOSFETs, and hence the standby power consumption by the MOSFETs, increases while they are turned off. It is obviously necessary to reduce the standby power as much as possible.
To attain this goal, normally a power switch is used which has a P-type MOSFET 11 and an N-type MOSFET 12 both having a high threshold voltage (HIGH Vth). With this switch, the transistor circuits in the block 10 are operated at a high frequency at the low threshold voltage (LOW Vth), but when they are in standby condition, the MOSFETs 11 and 12 are turned off at the high threshold voltage (HIGH Vth), thereby suppressing the leak currents (i.e., the standby power).
In this conventional IC device, the threshold voltage of the MOSFETs such as CMOSs of the circuit block 10 is set to a low level (LOW Vth) while the threshold voltages of the P-type MOSFET 11 and of N-type MOSFET 12 serving as a power switch must be set to a high level. In providing the MOSFETs 11, 12 with a threshold voltage higher than that of other MOSFETs, a special mask is needed to perform extra ion implantation in the process of manufacturing the IC device.